1. Field of the Invention
The present invention relates to an apparatus and method for improving transport control protocol (TCP) performance using path recovery notification over a wireless network, and more particularly, to an apparatus and method for path recovery notification which explicitly notifies recovery of a temporary disconnected wireless network in order to solve a problem in which TCP performance decreases due to a reduced data transfer rate when a congestion control algorithm is performed at a transmitting end because network congestion is misunderstood as a cause of packet loss while the packet loss is actually caused by a high probability of bit error or a handoff.
2. Description of the Related Art
In an attempt to overcome a problem in poor transport control protocol (TCP) performance caused by packet loss, not caused by network congestion, several techniques have been proposed such as U.S. Patent Application No. 2005-0185664 by Hemant Chaskar et al., U.S. Patent Application No. 2003-0117992 by Yun Joo Kim et. al, and Korean Patent Application No. 2004-0067343 by Chang Hwan Park.
First, U.S. Patent Application No. 2005-0185664 discloses a split TCP connection method in which TCP sections are divided into a wireless TCP section and a wired TCP section. Packet loss of the wireless TCP section is handled according to a wireless TCP of the wireless TCP section, so that poor performance can be prevented when the wired TCP section misrecognizes that the packet loss is caused by network congestion. However, the method has a disadvantage in that a new TCP session has to be connected when a mobile host performs a handoff to change a base station. Furthermore, in order to prevent this disadvantage, scalability of a network structure cannot be achieved.
In addition, U.S. Patent Application No. 2003-0117992 discloses a method in which a snoop agent of a base station analyzes a TCP packet without having to alter an end-to-end TCP and handles packet loss produced over a wireless network by using local retransmission. However, when a mobile host performs a handoff, a first snoop agent inconveniently has to pass a TCP data packet and state information stored in a buffer to a second snoop agent of a base station to which the mobile host belongs after the handoff is performed. Therefore, the method has a disadvantage in that, in this process, retransmission timeout (RTP) occurs in a TCP section of a transmitting end, which may lead to poor TCP performance.
In addition, Korean Patent Application No. 2004-0067343 discloses a method in which a snoop protocol is expanded so that the state of a wireless link is divided into a normal state, a bad state, and a handoff state. If the wireless link is in the normal state, performance is maximized by setting a maximum buffer size used in a snoop module to a congestion window size. In this case, in order to avoid network congestion, a congestion window of a transmitting end has to transmit data by selecting a smaller value between a receiver's window size and the congestion window size. However, when the congestion window is arbitrarily increased to be large in size, network congestion may occur in the wired network. On the other hand, if the wireless link is in the bad state, the snoop module transmits an acknowledgement (ACK) packet, in which the receiver's window size is set to 0, to a TCP section of the transmitting end when the RTO occurs two times, thereby preventing a transmitter's TCP section from transmitting a data packet. However, even in this case, reliable TCP performance is not guaranteed since the RTO of the transmitter's TCP section cannot be completely avoided. Finally, if the wireless link is in the handoff state, the snoop module avoids data loss caused by a handoff while passing its own data packet and state information to the snoop module of the base station to which a mobile host is moved. In this case, the snoop module of the base station has to handle packet forwarding. In general, the snoop module of the base station has to independently manage buffers for respective TCP connections. As a result, overhead occurs in the operation of the base station.
Meanwhile, in order to solve a problem of temporary disconnection caused by the handoff of the wireless link or a transmission interruption of the base station, a freeze-TCP has been proposed in academic circles. In the freeze-TCP, a TCP section of a receiving end predicts disconnection of the wireless link and sends a zero window advertisement to a TCP section of a transmitting end in advance. Therefore, a persist mode is continued while packet transmission is prevented, thereby avoiding packet loss in the wireless link. When the link is recovered, the TCP section of the receiving end informs a normal window size, so that the persist mode is released and thus the TCP section of the transmitting end can transmit a data packet without performance degradation. However, in practice, reliable performance is not expected when this method is used, since the performance can be guaranteed when disconnection time of the wireless link is accurately predicted.